[maemo-users] [maemo-users] Quick review of the Nokia 770

[From: David D. Hagood]

Lionel Dricot (aka Ploum) wrote:

>> > Why can't I have access to the MMC card when USB is plugged ?
>>
>> You (actually your PC) _has_ access to the MMC card, I thought that was
>> the whole point (apart from for flashing)
> 
> but why is concurrent access not possible?

Because of how USB mass storage devices and the FAT file system work.

USB storage devices just make a "pile of sectors" available to the host 
- so what is happening is that the Nokia is allowing the PC to have 
sector level access to memory stick.

Now, the first problem is that if the Nokia kept the file system mounted 
during this, and the PC wrote to the disk, then the data structures 
representing the file system to the VFAT layer in the Nokia would no 
longer match the actual file system - and you would get corruption.

Now, in theory the Nokia could flush its buffers every time the host 
wrote, but that only solves half the problem - if the Nokia wrote to the 
file system the the PC would have no idea the file system changed, nor 
is there any mechanism in the USB mass storage protocol to so notify it 
- and once again you have file system corruption.

You would have to force both parties to mount the file system read-only 
(not very useful).